There are differences wrt to ISRU such as CO2+nitrogen atmosphere, perchlorate soil, ice underground vs (probably) icy regolith containing CHON elements and more difficulty with solar power on Mars, but IMO these are extremely trivial compared to the hundred thousand things that must happen internally for such a city to not collapse. Also, it appears that both have the resources necessary to create methane/oxygen propellant.

You also can and should do a lot of this on earth. However there are some things you cannot finalise on earth. For most statistics the lunar poles are a more extreme problem than Mars. For most issues if your equipment works on both earth and the moon it will work on Mars. If Luna gravity proves sufficient for health then so is Mars gravity.

Although in most statistics the moon is harder, you could get home in 4 days. You could probably survive in your suit that long. Launch windows are frequent, landing is less scarily complex.

Would it be worthwhile to try building a full-blown working colony on the Moon, before risking sending many people to Mars? Or would it be an unnecessary distraction from focus on Mars?

Perhaps with a working lunar colony, it could expose any deeper problems that might not be exposable short of a full reality test. While this is in progress, the spaceflight operations supporting such a lunar colony could still iteratively improve to bring costs down in the meantime

As far as the lunar surface, there would be testing powered landings. Significant technologies needed for Mars, ISRU and aerocapture/aerobraking namely, would be impossible to test on Luna, but, so long as it is scaled for Mars' greater gravity, the propulsive system could be flown during Lunar missions.

Habitats could be tested as well; considering Mars' atmosphere is barely one percent Earth's what difference there is between Mars and Luna relate to the larger temperature variations on Luna. An inflatable habitat, for example, could handle both so long as it could handle the hot and cold spikes during a Lunar month; i.e. a short term test of a hab on the Moon would prove it could be stable on Mars.

There could be arguments that it's more cost effective to test such things strictly on Earth, but another way to look at it is how NASA's constantly forced back and forth between goals due to political whims...which will be inevitable despite efforts to minimize such madness. Employing vehicles and equipment that are adaptable to both environments ensures flexibility and redundancy. A Mars vehicle might be overpowered for Lunar standards, but the Moon is always more conveniently located.

Antarctica and Northern Canada would be much less expensive (and safer) places to test a lot of this stuff, except hab reliability in very low pressure. Everything else that is specific to Mars (ISRU chemistry) either can't be done on the Moon either, or can be simulated on Earth (such as living with 40 minute communication delays). Indeed, some people have already been doing this.

Powered descent and landing does not need to be tested on the Moon first; this is well understood technology on both Earth and Mars.

Having a million people live away from Earth and running profitable industries.

Colonizing space is not just about technology. It is also about business models and humans. The moon has far more immediately available business models or colonization schemes available to it than Mars does and is easier to monetize. Experience with running a profitable business on the moon will likely translate better into running a profitable venture on Mars than any single technology would.

For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v. Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Powered descent and landing does not need to be tested on the Moon first; this is well understood technology on both Earth and Mars.

Mars landing is radically different to a Moon landing, though some technologies are common - but not enough to justify test missions to the Moon on the way to Mars. The Moon remains, however, a worthwhile near-term goal - and probably a more politically acceptable one for state space agencies.

Testing semi closed habitats in low gravity, exposed to galactic radiation. near vacuum and dangerous dust. We need to prove the reliability of these habitats before sending them 24 months away from any spare parts or other help.

Yeah I think the important thing is that you are stuck somewhere that forces a moderate fraction of your budget to go to LS, self sufficiency and ISRU. In this case being stuck may actually be a good thing. It would trap some budget into actually being applied to space settlement despite politicians actively fighting money escaping to any useful technology development.

Antarctica and Northern Canada would be much less expensive (and safer) places to test a lot of this stuff, except hab reliability in very low pressure. Everything else that is specific to Mars (ISRU chemistry) either can't be done on the Moon either, or can be simulated on Earth (such as living with 40 minute communication delays). Indeed, some people have already been doing this.

Powered descent and landing does not need to be tested on the Moon first; this is well understood technology on both Earth and Mars.

Yes the landing part is fairly irrelevant to mars. At least it is not that hard or new so not a huge diversion.. apart from a whole separate vehicle. Mars landing really is different and hard.

I think people should always understand that for any space project, 99.99% of it always has to happen on earth. Look at Apollo for example. There would only be a moderate benefit to testing on the moon so it would have to be pretty cheap. Testing in both earth and zero gravity should be enough.

The big argument for the moon is that Mars is too expensive to begin. However.. constellation. If something with twice the performance of Altair and a fraction of the budget was suddenly pulled out of a hat I would say go for it, but the politics are so corrupt you would pretty much have to develop it in stealth mode. :p

I still like the lunar poles as a target but Im not a mars-firster, or a moon-firster. The moon might not lead to mars at all. After mastering the moon we might proceed to phobos and the asteroids. I also like the idea of a DSH in high lunar orbit and a small fleet of SEP tugs delivering asteroid samples regularly. I think that would be a much better way of mastering asteroid colonisation than going to an asteroid. It is much safer and would not tie you to just one sample when asteroids are so diverse, from solid iron to 20% volatile.

What certainly can be tested on moon: How people react on long term living in a low-g environment. Currently we have virtually zero experience what even living a week unter 1/6th g means to the body. If it works at 1/6th g, it certainly works at 1/3rd g.

I expect it to be better than zero-g, but who knows, how our body will react to it, unless we try it.

What certainly can not be tested on moon: Greenhouses. 14 days daylight, puts some plants under severe stress. 14 days night, and quite a lot of plants just die.

And a running food supply is rather important...

Habitats, rovers, and so on: If it works on the moon, it certainly works on mars.

In what ways could the Moon serve as a convenient testing ground for things that might be used on Mars?What things could be tested on the Moon before trying them out on Mars?

Almost nothing, IMO. Landing is deeply different. The environment for habs (and their life support and power systems) is too different (the moon has no convection effects, enormous temp swings, and maybe double the radiation load.) Gravity and regolith are different enough to rule out common rovers, space-suits, etc.

Basically the difference between Mars and the moon is about as much as between Mars and Earth, and Mars and LEO. So you can test the majority of systems on Earth or in orbit as well as you can on the moon.

Mars, Earth, the moon, LEO are all about equidistant from each other, in terms of unique environments versus common hardware.

(If there's not too much mixing from micrometeorite bombardment, the polar ice might preserve layers corresponding to the moon's history of asteroid and comet bombardment. In order. I would expect that would be staggeringly useful to planetary scientists.)

Yeah I think the important thing is that you are stuck somewhere that forces a moderate fraction of your budget to go to LS, self sufficiency and ISRU. In this case being stuck may actually be a good thing. It would trap some budget into actually being applied to space settlement despite politicians actively fighting money escaping to any useful technology development.

However, if you are "stuck" on the moon, it would mean that the program specifically didn't deliver those things.

Just as Constellation, as VSE, was originally meant to develop ISRU fuel technology, but instead quickly devolved into an equatorial base. Then devolved further to a handful of flags'n'footprints Apollo-on-steroids landings before being cancelled entirely.

A non-cancelled Constellation, stripped of everything useful while still somehow consuming all available funding. That's what "being stuck" means.

However, if you are "stuck" on the moon, it would mean that the program specifically didn't deliver those things.

Just as Constellation, as VSE, was originally meant to develop ISRU fuel technology, but instead quickly devolved into an equatorial base. Then devolved further to a handful of flags'n'footprints Apollo-on-steroids landings before being cancelled entirely.

A non-cancelled Constellation, stripped of everything useful while still somehow consuming all available funding. That's what "being stuck" means.

IMO, which I dont know if I can really justify past an impression, is that just doing sorties was never going to happen (again ). I think that at the point the goal slipped back from a base it was already in freefall. It is like we went from 105% to 95%, and on one level that might have looked like a small change but really it was the difference between incrementally growing our presence on the moon and doing nothing, not even getting there.

Consider just having altair for example. I can't absolutely argue they could not have done a few sorties and then collapsed. But being stuck imples it is politically sustainable. I think common sense and penny pinching would at some point allow landing one Altair within driving distance of the previous and calling that a good mission. At that point you have the situation of reuse of assets on the surface, such as the rovers, power and perhaps habitable section (if any were left behind, I forget).

If you can get to the moon regularly you cannot help but have a tiny bit extra that can go into infrastructure and saving costs by doing the actually useful things. It might not seem a lot but it could actually be an infinite amount more than the situation of a starving elephant in a room full of peanuts, which results in nothing but eventually a dead elephant and in the meanwhile not even a single spare peanut.

The Moon and Mars are sufficiently different that I don't think it would be efficient to reuse much technology between the two missions. So I agree that the argument of reuse of technology is not a good one. However, what I do see as useful is the experience gained in performing complex Lunar missions. That experience is what I believe will lead to a successful Mars mission.

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Akin's Laws of Spacecraft Design #1: Engineering is done with numbers. Analysis without numbers is only an opinion.